Experimental And Mechanism Study On The SO₂ Reduction By CO Based On Iron Sulfides For Sulfur Production

China is a large energy-consuming country,in which the industrial production process will produce a large amount of SO2,but the existing SO2 mainstream treatment technology is not in line with the requirements of the society’s green and sustainable development,and sulfur,as a high value-added and important chemical raw materials,the domestic resources are in short supply and rely heavily on imports.Using cheap and easily available CO to reduce SO2 to sulfur thereby realize resourceful desulfurization not only solves the pollution problem of SO2,but also alleviates the shortage of domestic sulfur resources,which is of great significance to the construction of a resource-saving and environment-friendly society in China.However,at present,the supported metal catalysts commonly used in CO reduction of SO2 need to be prevulcanized to form metal sulfides,causing the preparation process is complex and the vulcanization degree is not easy to control,which can easily lead to instability of catalytic activity.In addition,lacking of studies on the antioxidant properties of catalysts under oxygenated atmosphere leads to the relevant reaction mechanism and the action pathway of CO reduction of SO2 are not clear.To this end,this paper proposes to use a natural iron sulfidepyrite(FeS2),to prepare high-performance iron sulfide catalysts without the need for sulfidation,and investigates its catalytic activity and cycling stability,respectively,focusing on the evolutionary properties of the catalyst phase structure and the microscopic reaction paths during the reaction process,which reveals the mechanism of CO-catalyzed reduction of SO2 reaction based on iron sulfide.The antioxidant performance of modified iron sulfide catalysts under oxygenated atmosphere was investigated to elucidated the antioxidant mechanism,and a new idea of utilizing waste activated carbon to regulate the oxygen content in high oxygenated atmosphere was proposed and its feasibility was verified.The main results of this paper is concluded as following:In view of the problems of pre-sulfurization and complicated preparation process of traditional catalysts,the idea of preparing catalysts with natural iron sulfide(FeS2)is proposed.The effects of preparation conditions such as catalyst carrier,preparation method,particle size,and FeS2 content,as well as reaction conditions such as temperature,CO/SO2 molar ratio,and volumetric airspeed,on the catalytic activity were investigated using a fixed-bed reaction system.Under the conditions of particle size of 40～80 mesh,FeS2 content of 6 wt.%,CO/SO2 molar ratio of 2.6:1,reaction temperature of 550℃ and volumetric airspeed of 5000 h-1,the iron-sulfide catalyst was able to maintain more than 99.5%of SO2 conversion and about 95.5%of sulfur yield after 300 h of reaction.There was no deterioration of catalytic activity due to deactivation of the active component or accumulation of sulfate,which proved that the iron sulfide catalysts had excellent catalytic activity and stability.Aiming at the unclear reaction mechanism of CO reduction of SO2 catalyzed by iron-based metal sulfides,the phase structure evolutionary characteristics of the catalysts as well as the microscopic reaction paths during the reaction process have been investigated by using In-situ XRD,In-situ DRIFTS,FactSage and Density-functional theory.The results show that CO/SO2 molar ratio and volume airspeed affect the amount of two iron sulfides,FeS2 and Fe7S8,in the iron sulfide catalysts,while the temperature can control the form of iron sulfides present in the catalysts.Both FeS2 and Fe7S8 are active components of iron sulfide catalysts in the SO2 reduction reaction.The critical temperature for the reaction between FeS2 and CO is between 515 and 525℃,whereas Fe7S8 can react spontaneously with CO,and the catalytic activity of Fe7S8 is higher than that of FeS2 at temperatures less than 525℃.During the reaction,the process of S removal for iron sulfide catalysts is FeS2→Fe7S8→FeS,and the process of S gain for iron sulfide catalysts is the opposite.The reaction mechanism of iron sulfide catalysts includes two parts,namely,the mechanism of desulfurization by thermal decomposition of FeS2 and the COS intermediate product mechanism.Iron sulfide catalysts achieve catalytic activity through the dynamic transformation between three iron sulfides with different sulfur contents,FeS2,Fe7S8 and FeS,and the formation of Fe7S8 is the first step of iron sulfide catalysts to participate in the reduction reaction.In order to solve the problem that the catalyst is easy to be oxidized and deactivated and the mechanism of antioxidation is relatively lacking,this paper proposed to add the Fe monomers to make modified iron sulfides,and the antioxidant properties as well as the antioxidant mechanism of modified iron sulfide catalysts under oxygenated atmosphere were investigated.Density functional theory calculations confirm that O2 reacts more readily with Fe monomers compared to iron sulfides.The SO2 conversion and sulfur yield of the modified iron sulfide catalyst were maintained at about 99.9%and 93.1%.respectively,at a reaction temperature of 500℃,Fe/FeS2 molar ratio of 1:1(FeS2 content of 6 wt.%),CO/O2 molar ratio of 3:1,the volumetric airspeed of 6000 h-1,and O2 concentration of 3%.And the SO2 conversion remained stable at 97%～99%and the sulfur yield was maintained at 90%～93%after 5 cycles of the cycling experiment,indicating that the modified iron sulfide catalysts have excellent stability under an oxygen-containing atmosphere.During the reaction process,there are two source of Fe7S8 conversion in the modified iron sulfide catalysts,which are FeS2→Fe7S8 and Fe→Fe7S8.The essence of antioxidant mechanism of modified iron sulfide catalysts is a process in which Fe and O2 react to form Fe2O3,and then Fe2O3 is reduced to Fe by CO.In addition,a new idea of using waste activated carbon to regulate the oxygen content in the oxygenated atmosphere and enhance the CO content,and then catalyzing the reduction of SO2 by CO to prepare sulfur using the developed catalyst was proposed.The conditioned flue gas was obtained by reacting at a temperature of 700℃,an O2 concentration of 7%,an SO2 concentration of 1%,and a charcoal layer height of 30 mm.Then the modified iron sulfide catalyst was used to reduce the conditioned flue gas,the SO2 conversion rate was close to 100%,the sulfur yield was as high as about 94%,no oxidative deactivation of the catalyst was observed,thus the experimental results confirmed the feasibility of the idea.It was also revealed that the elemental S in the high-oxygen flue gas conditioning process is mainly recycled in the form of sulfu r,a very small amount goes into the conditioned flue gas in the form of COS and CS2,and a trace amount of elemental S is immobilized in the activated carbon in the form of metal sulfates.The paper reveals the reaction mechanism of CO reduction SO2 catalyzed by natural iron sulfide catalysts by studying the transformation characteristics of the physical phase structure and microscopic reaction paths,which solves the problem of unclear reaction mechanism of traditional metal sulfide catalyzing,and provides theoretical support for the development and application of CO reduction of SO2 technology using natural ores as catalysts.